This invention relates to modified polyamideimide resins of high solids content solutions with unexpectedly low viscosity.
It is known from Nakano et al., U.S. Pat. No. 3,541,038; Sheffer, U.S. Pat. No. 3,578,639; and Disque et al., U.S. Pat. No. 3,884,880 that high molecular weight polyamideimide resins can be formed by condensing one mole of a tribasic acid anhydride, such as trimellitic anhydride, with 1.00 to 1.06 moles of a diisocyanate compound, such as diphenyl methane diisocyanate, at a temperature of about 60.degree. C. to 150.degree. C. in the presence of an inert solvent.
These resins have good electrical insulation properties but begin to decompose, i.e., suffer loss in molecular weight, at temperatures below 300.degree. C.
It has now been found that the heat resistance of polyamideimide resins can be improved by partially substituting for the diisocyanate used in making such resins an isocyanate mixture containing a diisocyanate of the formula EQU OCN--R--NCO
and a triisocyanate of the formula ##STR1## or an isocyanate system in equilibrium comprising a diisocyanate of the formula EQU OCN--R--NCO
and its cyclotriisocyanate adduct having the formula ##STR2## which is prepared by reacting the diisocyanate with carbodiimide of the formula EQU OCN--R--N.dbd.C.dbd.N--R--NCO
in a manner known in this art. In all of the above formulae, R is an organic divalent radical of from 2 to 30 carbon atoms, optionally containing --O--, --SO.sub.2 -- and --S-- groups.
Preferably, R will be selected from the following radicals: ##STR3##
Further it is found that partial substitution, i.e., up to about 25 mole percent, of the diisocyanate with a di- and tri-isocyanate mixture or equilibrium system gives a polyamideimide resin having surprising polymer solution stability and unexpectedly low viscosity which maintains its flexibility as a cured film. This combination of solution stability at high solids contents, low viscosity and flexibility after curing makes the present invention especially useful as a coating or wire enamel.